1. Field of the Invention
This invention relates to ultrasonic Doppler diagnostic devices having a Doppler mode, and in particular, to a medical ultrasound device that uses multi-channel digital signal processing of a received continuous wave (CW) ultrasound echo signal for developing Doppler data.
2. Description of the Prior Art
The ultrasonic Doppler method is widely used for non-invasively detecting and measuring movement within a body, and finds wide use in medical ultrasound scanners for non-invasive diagnostic analysis of blood flow within a patient, e.g., for the detection and measurement of blood flow within the heart, blood vessels, etc., of a patient.
There are basically two operational modes of ultrasound Doppler; continuous wave (CW) and pulsed (PW). The PW mode is particularly useful for obtaining velocity data used to form a two-dimensional blood flow image (color flow image). However, because the pulse repetition rate (PRF) of PW Doppler systems limits the maximum flow velocity which can be determined without aliasing, the CW mode is found to be particularly useful for obtaining velocity data to accurately determine relatively high flow velocities.
Additionally, diagnostic ultrasound devices conventionally develop a so-called B-mode image, which is basically a two dimensional tomographic image, as well known to those skilled in the art. The B-mode image is formed using a transducer which can scan an area, and conventionally uses, as also well known in the art, one of several types of multiple element transducer arrays, such as a linear array of 64 or 128 transducer elements. On the other hand, CW Doppler operation generally requires the use of a special transducer probe which has two transducers, one for transmission and one for reception (as also well known in the art and commonly referred to as a pencil probe). It is desirable, for the sake of simplicity, to use a single probe and its receive signal processing circuitry to obtain the data useful for not only the Doppler mode, but also the B-mode. One technique to accomplish this is described in U.S. Pat. No. 4,915,115 entitled "Ultrasonic Imaging Apparatus for Displaying B-Mode and Doppler-Mode Images" and issued Apr. 10, 1990 to Sasaki et al. As described therein, data is obtained by first operating the transducer array via conventional transmit/receive control circuitry so as to provide steering and focusing of the transmitted ultrasound pulses and for providing appropriate individual delays to the received signals so that a conventional beamformer and image processor can create the B-mode image. Thereafter, the user can observe the B-mode image, and mark the image using a cursor with an indication of direction and depth from which PW Doppler data is desired to be obtained. Then, the transducer is operated in the PW Doppler mode so as to receive Doppler echoes and develop therefrom Doppler data. It is noted that the CW Doppler mode is not specifically described by Sasaki et al. Instead, Sasaki et al. is concerned with the reception efficiency of the transducer when it is desirable to operate it in both of the Doppler and B-mode, and solves this problem by providing an ultrasonic transducer having two peaks in its reception efficiency characteristic curves, one centered at the frequency used for B-mode imaging and the other centered at the frequency used for PW Doppler data gathering.
Using a multi-element transducer array for the CW Doppler mode is desirable because the transmit beam steering and reception focusing results in improved signal-to-noise (S/N) performance as well as an opportunity to better select the Doppler sample volume (the overlap regions between the CW transmit and receive beams). U.S. Pat. No. 4,598,589 entitled "Method of CW Doppler Imaging Using Variably Focused Ultrasonic Transducer Array" and issued Jul. 8, 1986 to Riley et al. describes operation of a multi-element transducer array for obtaining CW Doppler images, using conventional analogue signal processing techniques.
It is noted that the beamforming signal processing for B-mode imaging is starting to include digital techniques, while CW Doppler is currently carried out using analogue techniques. It would be desirable, however, to include a dual mode of operation for a multi-element transducer array which would allow for B-mode or CW Doppler operation while using the same digital beamforming techniques currently used for B-mode processing. Digital beamforming techniques are desirable because of the greater flexibility they offer under microprocessor control as compared to analogue circuits. However, a signal-to-noise (S/N) problem arises when trying to use digital processing for the CW Doppler echoes, which problem does not arise when processing PW Doppler or B-mode echoes. That is, in PW Doppler and B-mode imaging the conventional pulse operation, in combination with the range gating techniques, result in non-detection of large amplitude clutter signals which are caused by inter-transducer element crosstalk of the transmitted ultrasound signals and the reflection of the transmitted ultrasound signals from near-field reflectors. Since these signals are so much stronger than the desired signal, in the order of 80 to 100 dB, a problem exits in that the dynamic range of the echo digitization is currently limited to about 60 dB since such high speed (e.g., 36 mHz) A/D converters provide at most 10 bit output words. Furthermore, it is not technically and/or economically feasible to increase the dynamic range of the A/D converters in the foreseeable future to allow digital detection of the very low level Doppler shift signals in the presence of the very strong clutter signals.
One would think, however, that the known technique of signal dithering could be used to improve the dynamic range performance of the A/D converters. In this regard, one would expect that when multiple parallel channels are digitizing and processing received echoes to create the Doppler information signal, a different source of dithering signal would be required for each digitizing signal processing channel, so as to prevent the same dither signal from being added to each received signal and result in coherent addition of the dither signal in the finally formed beam. This requirement would result in a significant increase in circuitry, complexity and cost, to accomplish steerable CW Doppler.
It is an object of the present invention to allow a CW Doppler mode of operation for an ultrasound imaging apparatus which uses a multi-element transducer array.
It is a further object of the invention to allow the CW Doppler echo signals to be processed using the same digital beamforming circuitry used for the B- or PW Doppler modes of operation.
It is a still further object of the invention to provide such digital beamforming processing in a simple and low cost arrangement.
These and other objects of the invention will become apparent from the following detailed description of a preferred embodiment of the invention, the accompanying drawings and the claims.